Conventional power equipment includes the steam engines, internal combustion engines, and external combustion engines.
Steam engine: It cannot work without the boiler. The whole machine is heavy and large. The pressure and the temperature of the new steam cannot be too high, and the exhaust pressure cannot be too low. The heat efficiency is hard to improve. It is a reciprocating machine. The inertia restrains the improvement of the rotational speed. The working is not continuous. The flow of steam is restrained, which limits the improvement of the power.
Internal combustion engine: It has a complicated structure, a high requirement of fuel, and strict requirement of the cleanliness of fuel. It pollutes the environment.
External combustion engine: For example, Stirling engine is one kind of external combustion engine. Compared with internal combustion engine, Stirling engine has the following advantages:
It is suitable for all kinds of energy, not matter what state the energy is derived from, liquid, gas, or solid fuel. When using the heat-carrying system (e.g., heat pipe) to heat indirectly, almost all high-temperature heat source (e.g., solar radioactive isotope and nuclear reaction) can be used, while the engine itself (except the heater) does not need any change. At the same time, Stirling engine does not need a compressing machine to increase the pressure, which can be met by an ordinary fan. The fuel with relatively high impurity content is allowed. The unit capacity of Stirling engine is small, the capacity of which ranges from 20 to 50 kw. The system capacity can be increased or reduced according to local conditions. The structure is simple. The number of parts of the external combustion engine is 40% less than that of an internal combustion engine. It has a significant margin of price discount and a low maintenance cost.
When Stirling engine is running, the fuel is burning continuously in the combustion chamber which is outside the cylinder. The working medium, which is independent from the gas, absorbs the heat from the heater and works to the outside environment according to the Stirling cycle. Thus, the engine knocking and intermittent combustion of the internal combustion engine and the like are avoided. An efficient, less noisy, and low-exhaust operation is realized. As being efficient, the total energy efficiency reaches more than 80%. As being less noisy, the noise at a place which is one meter from the bare machine is lower than 68 dBA. As being low-exhaust, the emission of tail gas meets the standard of Euro 5.
Since the working medium does not burn, the external combustion engine avoids the problem of knocking of the conventional internal combustion engine, such that high efficiency, low-noise, low-polluting, and low-running-costs are realized. The external combustion engine can burn various gases, such as natural gas, biogas, petroleum gas, hydrogen, gas, etc. Liquid fuels like diesel, liquefied petroleum gas, etc. can also be used. Burning woods, the solar energy, etc. can also be used. As long as the temperature of the hot chamber reaches 700° C., the equipment will run and work. Lower the environmental temperature, the higher will be the efficiency of the power generation. The most remarkable advantage of the external combustion engine is that the output and efficiency are not affected by the altitude, which makes it very suitable for using in high-altitude areas.
Meanwhile, the mainly existing problems and defects of Stirling engine are as follows: the manufacturing cost is high; the working medium sealing technology is difficult the reliability and serving life of the sealing part have problems; the material cost is high; the power adjusting control system is complex; the machine is heavy; the costs of the expansion chamber, the compression chamber, the heater, the cooling chamber, the regenerator, etc. are high; and the heat loss is twice to three times than that of an internal combustion engine.
Organic Rankine Cycle system includes a pump, an evaporator, an expander, a generator, a condenser, etc. The heat collector absorbs the solar radiation. The temperature of the heat exchanging medium inside the heat collector increases. The heat is transmitted to the organic working medium from the heat exchanging medium through the evaporator. The organic working medium is heated in the evaporator under a constant pressure. The gaseous organic working medium with a high pressure enters the expander to work through expanding, so as to drive the generator to generate power. The organic working medium discharged from the tail of the expander enters the condenser to condense under a constant pressure. After increasing the pressure by the pump, the organic working medium output from the condenser enters the evaporator, such that one power generation cycle is completed.
Organic Rankine Cycle system has the following defects. The conversion efficiency is low. The size is huge. The expander which has a complex structure is essential to work.